TITLE: GCN CIRCULAR NUMBER: 40015 SUBJECT: GRB 250402A: SVOM/VT optical counterpart DATE: 25/04/03 00:26:36 GMT FROM: Liping Xin at NAOC, SVOM xlp@nao.cas.cn

L.P. Xin, Y. L. Qiu, C. Wu, H. L. Li, J . Z. H. Yao, Y. Xu, P. P. Zhang, W. J. Xie, J. Wang, X. H. Han, Y. J. Xiao, H. B. Cai, Y. N. Yi, J. Y. Wei (NAOC), J. T. Palmerio(CEA/Irfu), T. Sadibekova(CEA) report on behalf of the SVOM team:

SVOM performed an automatic slew on the burst triggered by SVOM/ECLAIRs (Chen et al., GCN 40014). SVOM/VT began observing the field automatically with the slew of the platform triggered on-board, in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.

With VHF downlinked data, a bright uncatalogued source was found in both channels in the first sequence, and brightening with about 0.2 mag in the second sequence, compared to DESI catalog.

The coordinates are RA, Dec = 213.40915, -5.93013 which is equivalent to:

R.A.(J2000) = 14:13:38.20, DEC.(J2000)= -05:55:48.5 Error=0.5 arcseconds.

The brightness are VT_B ~21.53 mag(AB) and VT_R ~20.61 mag(AB) in the first sequence with the mid time of 443 seconds after the burst.

We proposed that this is the optical counterpart of the burst. Given the VT color, it is likely a low/medium redshift burst.

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC),CAS.

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